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The gravel trail surface is too hard to soak up rainwater, so the runoff flows off the gravel instead and into the linear rain garden on the left.

10th St. and Mill Ave. Rain Garden

The gravel trail surface is too hard to soak up rainwater, so the runoff flows off the gravel instead and into the linear rain garden on the left.

Where is this located? It’s at the parking lot at the end of 10th Street and adjacent to the South Bay Trail.

What’s special about this place? While beautifying the area, the wild roses and grass roots keep the soil loose so that water from the parking lot and trail can soak in. It’s possible you’ll see chickadees, towhees, and other small birds foraging for bugs, seeds, and worms in the protection of these plants. Most of the water soaks into the soil. But if there is too much rain, a drain below the soil carries the extra runoff downhill to the 8th Avenue Bioretention Basin. This technique serves a double purpose of providing safe separation of cars in the parking lot from users on the trail.

About Rain Gardens

A rain garden is a bowl-shaped garden that collects, absorbs, and filters the rainwater that runs off hard surfaces including parking lots, driveways, sidewalks, lawns, and roofs. They provide on-site cleaning of stormwater by using special soil mixtures and low-maintenance plants that soak up runoff. The soil mixes used in rain gardens are selected to allow water to soak in rapidly, treat runoff, and support plant growth. Hardy, often native, plants are carefully selected for each site, including wildflowers, grasses, rushes, ferns, shrubs, and even small trees. These plants can have the added benefit of attracting pollinators, providing habitat for beneficial insects and birds, adding beauty, and improving neighboring property values.

Rain gardens can be small and home-made, or they can be engineered with specific materials to be highly effective. Home-made rain gardens are usually applied in smaller, residential systems working to intercept runoff that would otherwise flow onto streets. Engineered rain gardens are a type of bioretention basin and are sized for specific water quality treatment and flow control capacity that includes designed soil mixes and, often, under-drains and control structures. All the rain gardens highlighted on these tours are engineered bioretention facilities.

Water Quality Benefit: Rain gardens can capture sediment and bacteria, capture oil, pesticides, and fertilizers, and use their mulch layer to bind-up dissolved metals. Plant roots also are host to diverse microbial and fungal populations that filter pollution using biological processes.

Water Quantity Benefit: Special soil mixtures are chosen to allow water to soak into the ground. Roots break up the soil so water can soak into the ground more easily. By absorbing runoff from hard surfaces, rain gardens reduce flooding on neighboring property, reduce erosion in streams, and may recharge local groundwater.

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Drains over half an acre.

Where does this stormwater runoff come from? Stormwater at this facility comes from the entire parking lot at the end of 10th Street, the nearby building, and the West Bay Trail.

Where does this stormwater runoff flow next? Runoff that doesn’t infiltrate into the ground flows through pipes under Harris Avenue where most of the water is sent to the 8th Street Bioscape. During high flows some water goes directly to Padden Creek.

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9th St. and Harris Ave. Vault
South Bay Trail Rain Garden